Wide band gap transistors on non-native semiconductor substrates and methods of manufacture thereof

ABSTRACT

Techniques are disclosed for forming a GaN transistor on a semiconductor substrate. An insulating layer forms on top of a semiconductor substrate. A trench, filled with a trench material comprising a III-V semiconductor material, forms through the insulating layer and extends into the semiconductor substrate. A channel structure, containing III-V material having a defect density lower than the trench material, forms directly on top of the insulating layer and adjacent to the trench. A source and drain form on opposite sides of the channel structure, and a gate forms on the channel structure. The semiconductor substrate forms a plane upon which both GaN transistors and other transistors can form.

This patent application is a U.S. National Phase Application under 35U.S.C. 371 of International Application No. PCT/US2013/077621 filed Dec.23, 2013.

TECHNICAL FIELD

Embodiments of the present invention relate generally to wide band gaptransistors on semiconductor substrates and their methods offabrication. More particularly, embodiments of the present inventionrelate to gallium nitride transistors on silicon substrates and theirmethods of fabrication.

BACKGROUND

Power management (PM) and radio frequency (RF) amplification arecritical device processes performed in the operation of modern mobilecomputing platforms, such as smartphones, tablets, and laptop/notebooks.Integrated circuits (IC) contained in System-on-Chip products anddesigned to perform these operations, such as power managementintegrated circuits (PMIC) and radio frequency integrated circuits(RFIC), require transistors that can withstand high voltages andelectric fields. Typical voltages encountered by PMICs and RFICs thatperform high-voltage switching for DC-to-DC conversion in the outputfilter as well as in the drive circuitries, for example, can be as muchas 3.7 V as outputted by ordinary lithium batteries. Using silicontransistors to perform at these high voltages, however, proves difficultdue to the low band gap of silicon (i.e., 1.12 eV). For instance, inorder for a silicon transistor in a silicon-based PMIC to withstandvoltages of 3.7 V, the transistor size will need to be in the dimensionof tens of millimeters. In an alternative solution, silicon transistorsin a PMIC can be formed in series. However, such configurations havesignificant power losses and high resistances, which lead to shortbattery life and cooling issues. As a result, current solutions utilizealternative semiconductor materials with wider band gaps. One suchmaterial is gallium nitride (GaN).

GaN is a wide band gap (i.e., 3.4 eV) semiconductor material that hasbeen widely explored for its beneficial properties relating tomicro-electronic devices including, but not limited to, transistors,light emitting diodes (LED), and high-power integrated circuits. GaN hasa wurtzite crystalline structure with a lattice constant that is smallerthan the lattice constant of silicon, and has an electron mobilitysimilar to that of silicon, which is approximately 1300 cm² (v·s)⁻¹.

Currently, GaN is grown heteroepitaxially on non-GaN substrates by bruteforce (e.g., direct growth of epitaxial GaN on non-GaN substrates).Brute force growth of GaN on non-native substrates results insubstantial lattice mismatch between the substrate and the epitaxiallayer caused by the difference in their lattice structures and/orlattice constants. Lattice mismatch between a non-GaN substrate and aGaN epitaxial layer causes threading dislocation defects to propagate inall directions from the interface between the GaN epitaxial layer andnon-GaN substrate.

In an effort to decrease the amount of these defects, conventionalsolutions grow a thick buffer layer (e.g., greater than 1 μm) of GaN ona non-native substrate (e.g., silicon, sapphire, or silicon carbide) inhopes that a number of threading dislocations will cease to occursomewhere in the middle of growth. Even with several microns of bufferGaN growth, however, the defect density of resulting GaN cannot achievea defect density less than 2E7 cm⁻². Furthermore, the buffer layercreates a large height difference between GaN transistors formed on topof the buffer layer and other transistors formed on the siliconsubstrate, such as complementary metal oxide semiconductors (CMOS). As aresult, this height difference precludes direct heterogeneousintegration of GaN transistors on silicon substrates for co-integrationwith silicon CMOS transistors on the same substrate plane.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an isometric view of a wide band gap transistorco-integrated with a silicon-based finFET transistor on a siliconsubstrate, in accordance with an embodiment of the invention.

FIGS. 2A-2K illustrate isometric views of a method of forming a wideband gap transistor co-integrated with a silicon-based finFET transistoron a silicon substrate, in accordance with an embodiment of theinvention.

FIG. 3 illustrates a computing system implemented with oneimplementation of the invention.

DETAILED DESCRIPTION

Wide band gap transistors formed on non-native semiconductor substratesand their methods of fabrication are disclosed. Embodiments of thepresent invention are described with respect to specific details inorder to provide a thorough understanding of the invention. One ofordinary skill in the art will appreciate that embodiments of theinvention can be practiced without these specific details. In otherinstances, well known semiconductor processes and equipment are notdescribed in specific detail in order to not unnecessarily obscureembodiments of the present invention. Additionally, the variousembodiments shown in the figures are illustrative representations andare not necessarily drawn to scale.

Embodiments of the invention are directed to wide band gap transistorsformed on semiconductor substrates. Wide band gap transistors are formedby lateral epitaxial overgrowth (LEO) from an adjacent trench. Using LEOto form a channel structure for a wide band gap transistor precludes theneed for a large buffer layer. In embodiments, the wide band gaptransistors are co-integrated with silicon transistors on the same waferplane of a monocrystalline silicon substrate.

In one embodiment of the invention, a monocrystalline silicon substrateis provided. The silicon substrate includes a top silicon dioxideinsulating layer. A channel structure is formed directly on top of thetop silicon dioxide insulating layer of the silicon substrate. Thechannel structure is composed of a wide band gap semiconductor material.A gate electrode, a gate dielectric, and optional gate spacers areformed on top of the channel structure. Disposed directly adjacent tothe channel structure are a source and a drain. The source and drain aredisposed on opposite sides of the channel structure. Directly below thesource is a trench that extends from a top surface of the siliconsubstrate, through the silicon dioxide insulating layer, and into thesilicon substrate. Accordingly, the channel structure is adjacent to thetrench. The trench contains a trench material composed of a defectivewide band gap semiconductor material. The trench material and thechannel structure are composed of the same semiconductor material.However, the channel structure has significantly less defects than thetrench material. The trench material is thermally coupled with thesource, thus providing a direct heat sink to the substrate. Forming thechannel structure directly on top of the silicon dioxide insulatinglayer allows the wide band gap transistor to be co-integrated with atransistor on the same semiconductor substrate.

FIG. 1 illustrates an isometric view of a wide band gap transistorco-integrated with a finFET transistor on a silicon semiconductorsubstrate 102 in accordance with an embodiment of the present invention.Parts of the wide band gap transistor and finFET device, such as firstcontacts (e.g., source and drain contacts) and an interlayer dielectric,are not shown for purposes of clarity. An illustration in region A ofFIG. 1 depicts wide band gap transistors formed on a semiconductorsubstrate 102. Additionally, an illustration in region B of FIG. 1depicts a finFET transistor formed on the semiconductor substrate 102 inthe same wafer plane as the wide band gap transistors. As shown in FIG.1, the wide band gap and finFET transistors are co-integrated with oneanother. That is, the wide band gap transistor and the finFET transistorare formed side-by-side within the same wafer plane. The dotted lines inbetween the two structures indicate that the finFET transistor is formedon the same wafer plane as the wide band gap transistor regardless ofwhether the wide band gap transistor is adjacent to or far away from thefinFET transistor.

Semiconductor substrate 102 may be composed of any suitable substratefor semiconductor device fabrication, such as a bulk monocrystallinesilicon substrate. The semiconductor substrate 102 includes a dielectriclayer 104 formed on a top surface of the semiconductor substrate 102. Assuch, the dielectric layer 104 electrically isolates the semiconductorsubstrate 102. Any suitable dielectric material, such as silicondioxide, may be used to form the dielectric layer 104.

As depicted in region A of FIG. 1, a channel structure 106 is disposedon the dielectric layer 104 of the silicon substrate 102. The channelstructure 106 is composed of a wide band gap semiconductor material. Insome embodiments, the channel structure 106 is composed of asemiconductor material with a band gap greater than 2.0 eV. In otherembodiments, the channel structure is composed of a III-V material. Inone particular embodiment, the channel structure 106 is composed of GaN.The dielectric layer 104 electrically isolates the channel structure 106from the silicon substrate 102. Electrically isolating the channelstructure 106 from the substrate 102 advantageously reduces transistorbody leakage and parasitic capacitance. Furthermore, using a siliconsubstrate 102 with a dielectric layer 104 advantageously precludes theneed for an expensive silicon-on-insulator (SOI) substrate.

The channel structure 106 may have a small amount of defects within itscrystalline structure. Defects within the crystalline structure of thechannel structure 106 increase channel resistance when the wide band gaptransistor is turned ON. The increased channel resistance causes thetransistor to operate inefficiently. As such, a channel structure 106with a low defect density is desired. In one particular embodiment, thedefect density of the channel structure 106 is lower than 1E9 cm⁻² dueto the use of lateral epitaxial overgrowth (LEO). In an alternativeembodiment, the defect density of the channel structure 106 is lowerthan 2E7 cm⁻².

Disposed directly adjacent and on opposite sides of the channelstructure 106 are a source 108 and a drain 110. The source and drain 108and 110 are composed of any suitable semiconductor material that can beepitaxially grown from the channel structure 106. For instance, asuitable semiconductor material is an alloy of the channel structure106. In an embodiment, the source and drain regions 108 and 110 arecomposed of a material that has a band gap narrower than the channelstructure 106. As such, the contact resistance between the source anddrain 108 and 110 and a first contact can be minimized. In oneparticular embodiment, the channel structure 106 is composed of GaN andthe source and drain 108 and 110 are composed of indium gallium nitride(InGaN). Indium Nitride has a band gap of 0.9 eV. Thus, when indium isalloyed with GaN, the resulting overall effective band gap of InGaN islower than 3.4 eV. Moreover, because InGaN is an alloy of GaN, InGaN canbe epitaxially grown from the channel structure 106.

In some embodiments, the drain 110 is formed directly on the top surface105 of the dielectric layer 104. Accordingly, the source 108 is formeddirectly on a top surface 123 of a trench material 115 formed within atrench 107. As such, the trench material 115 is also adjacent to thechannel structure 106. Forming the source 108 directly on the topsurface 123 of the trench material 115 thermally couples the source 108with the substrate 102. Thermal coupling between the source 108 and thesubstrate 102 advantageously provides direct heat sinking to thesubstrate 102. Although the source 108 forms on the trench material 115in some embodiments, other embodiments may switch the locations of thesource and drain 108 and 110 such that the drain 110 is thermallycoupled with the substrate 102.

In embodiments, the trench material 115 is composed of the samesemiconductor material as the channel structure 106. For example,channel structure 106 and the trench material 115 both comprise a III-Vmaterial such as GaN. Although the channel structure 106 and trenchmaterial 115 may be composed of the same semiconductor materials, theamount of threading dislocation defects 117 within the channel structure106 is significantly less than the amount of threading dislocationdefects 117 within the trench material 115. For instance, both thechannel structure 106 and trench material 115 may be composed of GaN,but the channel structure 106 and has a defect density less than 1E9cm⁻² while the trench material 115 has a defect density greater than 1E9cm⁻². The channel structure 106 has a defect density lower than thetrench material 115 because the existence of defects within the channelstructure 106 may decrease transistor efficiency and reliability. Trenchmaterial 115 can have significantly more defects than the channelstructure 106. Under typical transistor operating conditions, thesubstrate 102 and the source 108 are equipotential. Accordingly, thetrench 115 and the source 108 are also equipotential. Thus, current doesnot pass through the trench material 115. As such, the high defectdensity of threading dislocations 117 in the trench material 115 mayhave little effect on transistor operation. In other embodiments, thetrench material 115 is composed of a different semiconductor materialthan the channel structure 106. For example, the channel structure 106may be composed of GaN and the trench material 115 may be composed ofInGaN.

The trench material 115 may be formed on a top surface 103 of thesilicon substrate 102. Any suitable epitaxial growth process may formthe trench material 115 on the top surface 103 of the silicon substrate102. In some embodiments, the top surface 103 of the silicon substrate102 may be a modified surface to aid in the epitaxial growth of thetrench material 115. By way of example, not by way of limitation, thetop surface 103 of the silicon substrate 102 may be modified to have aV-groove profile. The V-groove profile has modified top surfaces 103that expose the <111> plane within a global <100> silicon substrate andconverge at a lowest point. Compared to a flat-surface profile, theV-groove profile arranges silicon cubic crystals at the top surface 103in an orientation that allows for better crystalline matching duringepitaxial growth.

A gate electrode 112 forms on the channel structure 106. In someembodiments, the gate electrode 112 is a polysilicon gate electrode. Inother embodiments, the gate electrode 112 is a metal gate electrode. Adielectric layer 113 is disposed in between the gate electrode 112 andthe channel structure 106. In addition, a pair of gate spacers 114 mayoptionally be formed on opposite sides of the gate electrode 112.

As depicted in region B of FIG. 1, a semiconductor device, such as afinFET transistor, may be formed on the same wafer plane as a wide bandgap transistor. The finFET transistor may be a silicon-based transistor.The finFET transistor is formed on the substrate 102. The substrate 102includes a dielectric layer 104 located at the top surface 105 of thesilicon substrate 102 to electrically isolate the silicon substrate 102.A fin 101 extends through the dielectric layer 104 from the siliconsubstrate 102 in order to expose the top surface 143 and portions ofsemiconductor sidewalls 144 and 145 of the fin 101. A gate electrode 140wraps around the three exposed surfaces of the fin 101. A gatedielectric 142 is disposed in between the fin 101 and the gate electrode140.

Certain embodiments of the present invention may be fabricated accordingto the processes described with respect to FIGS. 2A-2K. FIGS. 2A-2Ddepict a wide band gap transistor region A and a finFET transistorregion B as the processes are performed. FIGS. 2E-2K depict only thewide band gap transistor region A as the processes continue to beperformed.

In FIG. 2A, a semiconductor substrate 202 with a patterned photoresistmask 226 is provided. The semiconductor substrate 202 can be composed ofa material suitable for semiconductor device fabrication. In oneembodiment, the semiconductor substrate 202 is a monocrystallinesemiconductor substrate. Semiconductor substrate 202 may also be, butnot limited to, silicon (Si), sapphire (Al₂O₃), silicon carbide (SiC),gallium arsenide (GaAs), and gallium phosphide (GaP). In one particularembodiment, the substrate is a global <100> oriented monocrystallinesilicon substrate. The photoresist mask 226 may be patterned on thesemiconductor substrate 202 to allow uncovered areas of thesemiconductor substrate 202 to be etched away. In addition to thephotoresist mask 226, an intervening hard mask may first be patterned tobetter resist mask degradation during the etching away of thesemiconductor substrate 202. In an embodiment, the photoresist mask 226is patterned in the finFET transistor region B to define locations wherefins for finFET transistors are to be formed. In another embodiment, thephotoresist mask 226 is patterned in the wide band gap transistor regionA to define locations where trenches are to be formed and where wideband gap semiconductor material is to be subsequently grown.

Next, in FIG. 2B, fins 201 are formed by etching away the uncoveredareas of the semiconductor substrate 202. A bottom surface 227 lies inbetween the fins 201. Each fin 201 has a top surface 243 and a first andsecond semiconductor sidewall 244 and 245. Fins 201A and bottom surfaces227A are formed in the wide band gap transistor region A, whereas fins201B and bottom surfaces 227B are formed in the finFET transistor regionB. Although three fins 201 are shown in FIG. 2B, it is noted that manymore fins 201 may be formed according to additional embodiments of theinvention. The fins 201 may be substantially rectangular, but otherembodiments are not so limited. The fins 201 can be formed by anysuitable anisotropic etch process, such as a plasma etch process using aCl₂-based process gas mixture. The photoresist mask 226 may be removedduring the formation of fins 201. In some embodiments, fins 201A and201B are formed simultaneously with one etch process. As such, the fins201A and 201B may be substantially similar to one another in both shapeand size. In alternative embodiments, fins 201A and 201B are formedseparately with at least two different etch processes. As such, thebottom surfaces 227A may be deeper than the bottom surfaces 227B tocompensate for a wide band gap transistor device height.

As shown in FIG. 2C, a shallow trench isolation (STI) layer 204 is thenformed on the bottom surface 227 located on either side of the fins 201.The STI layer 204 may be any suitable dielectric material, such assilicon dioxide. To form the STI layer 204, one deposition process maysimultaneously blanket deposit the dielectric material in the wide bandgap transistor region A and the finFET region B. Any well-knowndeposition process may blanket deposit the dielectric material, such as,but not limited to, chemical vapor deposition (CVD) or plasma-enhancedchemical vapor deposition (PECVD). After blanket depositing thedielectric material, the dielectric material may be planarized andsubsequently recessed to form the STI layer 204. Any suitableplanarization process, such as a chemical-mechanical polishing (CMP)process, may be used to planarize the dielectric material and anysuitable etching process, such as an HF wet etch process, may be used torecess the dielectric material to form the STI layer 204. Afterformation of the STI layer 204, only a portion of the semiconductorsidewalls 244 and 245 are exposed. The STI layer 204 provides anisolating layer that may be used to isolate a gate electrode from thesubstrate, as well as provide isolation between individual transistors.

Next, in FIG. 2D, the fins 201A in the wide band gap transistor region Aare etched selective to the STI layer 204. Any suitable etch processthat etches silicon but does not substantially etch silicon dioxide maybe used to remove the fins 201A. Trenches 207 form within thesemiconductor substrate 202 after performing the selective etch process.The trenches 207 extend through the STI layer 204 and into thesemiconductor substrate 202 to expose top surfaces 203 of thesemiconductor substrate 202. As depicted in FIG. 2D, an additional etchprocess may form a modified top surface 203 of the semiconductorsubstrate 202. The modified top surface 203 may include a V-grooveprofile that is formed by any typical crystallographic etch process. Inone embodiment, the modified top surface 203 is formed by a wet etchprocess with an active solution such as, but not limited to, potassiumhydroxide (KOH) or tetramethyl ammonium hydroxide (TMAH).

While some embodiments use fins 201A to form the STI layer 204 in thewide band gap region A, alternative embodiments may use a deposition,polish, and etching technique instead. For example, a dielectricmaterial may initially be blanket deposited on the semiconductorsubstrate 202. Thereafter, the deposited dielectric material may beplanarized to form a dielectric layer. Subsequently, areas of thedielectric layer where the trench 207 is to be formed may be etched toreveal the semiconductor substrate 202. As a result, a patterned STIlayer 204 is formed. It is to be appreciated that any other method offorming a patterned dielectric layer may be envisioned by embodiments ofthe present invention.

Although the fins 201A in the wide band gap transistor region A areselectively etched away, the fins 201B in the finFET transistor region Bmay remain as part of a finFET transistor structure. In one embodiment,the finFET transistor is part of a CMOS circuit. A gate dielectric 242is disposed on a portion of exposed surfaces of the fin 201B. A gateelectrode 240 is formed directly on top of the gate dielectric 242. Assuch, the gate dielectric 242 is disposed in between the gate electrode240 and the fin 201B. Any well-known deposition and etch processes maybe used to form the gate dielectric 242 and gate electrode 240. Althoughshown as a complete structure in FIG. 2D, the finFET transistor inregion B may be formed by processes before, during, or after theformation of the wide band gap transistor in region A. Any well-knownprocesses for forming finFET transistors may be used to form the finFETtransistor in region B.

Next, in FIG. 2E, a semiconductor material 216 is epitaxially grown fromthe top surface 203 of the semiconductor substrate 202 by any suitableepitaxial growth process, such as vapor-phase epitaxy (VPE), molecularbeam epitaxy (MBE), or chemical vapor deposition (CVD). In anembodiment, the semiconductor material 216 is composed of a wide bandgap material (e.g., any material with a band gap greater than 2.0 eV), aIII-V material, or any material that suffers from dislocations andstacking faults in its crystal structure during epitaxial growth on anon-native substrate. In one embodiment, the semiconductor material 216is GaN. In one particular embodiment, the semiconductor material 216 isGaN and the non-native substrate 202 is silicon.

Semiconductor material 216 initially epitaxially grows within theconfined boundaries of the trench 207 forming a trench material 215.Accordingly, semiconductor material 216 cannot grow laterally. As such,semiconductor material 216 grows substantially vertically (i.e., in the<0001> direction) within the trench 207. Threading dislocation defects217 may form in the semiconductor material 216 during epitaxial growth.These defects are caused by a lattice mismatch between the semiconductormaterial 216 and the non-native substrate 202. A non-native substrate isany substrate that has a mismatching lattice structure and/or amismatching lattice constant with the semiconductor material epitaxiallygrown from it. Threading dislocation defects 217 originate from the topsurface 203 of the semiconductor substrate 202 and propagate through thesemiconductor material 216 predominantly in the vertical direction.Horizontally and diagonally propagating threading dislocation defectsterminate against the sidewall 209 of the trench 207. As such, very fewhorizontally and diagonally propagating threading dislocation defectscontinue to propagate above the top surface 205 of the semiconductorsubstrate 202. Rather, only vertically propagating defects continue topropagate above the top surface 205. In embodiments, the defect densityof the trench material 215 is greater than 1E9 cm⁻².

The modified V-groove profile of the top surface 203 of the substrate202 aids in the epitaxial growth of semiconductor material 216. Comparedto a flat <100> surface profile, the V-groove profile arranges siliconcubic crystals at the top surface 103 in an orientation that allows forbetter crystalline matching with GaN wurtzite crystals during epitaxialgrowth. Better crystalline matching advantageously reduces the negativeeffects of lattice mismatch between the two crystalline structures. Inone embodiment, the V-groove profile decreases lattice mismatch to 17%from 41% as seen in growth on a flat <100> surface profile.

When the semiconductor material 216 grows above the top surface 205 ofthe STI layer 204, the semiconductor material 216 grows laterally (i.e.,in the <100> direction) onto the top surface 205 by lateral epitaxialovergrowth (LEO). Laterally grown semiconductor material 206 may havevery little threading dislocation defects 217 because most of thedefects 217 that propagate horizontally and diagonally have alreadyterminated into the sidewall 209 of the trench 207. As such, thelaterally grown semiconductor material 206 disposed on the top surface205 of the STI layer 204 is substantially high-quality material that issignificantly free of defects (“defect-free”). In one embodiment, thedefect density of the defect-free laterally grown semiconductor material206 is less than 1E9 cm⁻². In an alternative embodiment, the defectdensity of the defect-free laterally grown semiconductor material 206 isless than 2E7 cm⁻².

Laterally grown semiconductor material 206 extends a distance 218 overthe top surface 205 at one point in its LEO. As laterally grownsemiconductor material 206 continues to LEO, side surfaces 219 propagatein the <100> direction and extend toward an adjacent laterally growingsemiconductor material 206 until the side surfaces 219 of adjacentsemiconductor material coalesce and form a blanket layer ofsemiconductor material 216 and 206.

Referring now to FIG. 2F, a seam 224 is formed at the point where twoside surfaces 219 of adjacent semiconductor material coalesce. Thepressure exerted at the seam 224 from adjacent laterally overflowingmaterials results in defects formed in the area 222 around the seam 224.As such, the blanket layer of semiconductor material 216 and 206contains defective areas 221 and 222 and defect-free areas 220. In someembodiments, the defect density of semiconductor material 216 in thedefective areas 221 and 222 is greater than 1E9 cm⁻², whereas the defectdensity of the defect-free semiconductor material 206 in the defect-freeareas 220 is less than 1E9 cm⁻². Accordingly, the defect-freesemiconductor 206 has a significantly lower defect density than thetrench material 215. After forming a blanket layer of semiconductormaterial 216 and 206, the blanket layer of semiconductor material 216and 206 is subsequently planarized. Any suitable planarizing process,such as CMP may be performed to planarize the blanket layer ofsemiconductor material 216 and 206 if desired.

As shown in FIG. 2G, an insulating layer 228 is formed directly on topof the blanket layer of semiconductor material 216 and 206. Theinsulating layer 228 may be composed of any suitable dielectric such assilicon dioxide. Any well-known deposition technique may be used to formthe insulating layer 228. The insulating layer 228 isolates the topsurface of blanket layer of semiconductor material 216 and 206 fromexposure to subsequent process conditions.

Next, as illustrated in FIG. 2H, the defective portions of the blanketlayer of semiconductor material 216 and 206 in areas 221 and 222 areetched to form channel structures 206 and trenches 207 containing thetrench material 215. As a result, the top surface 205 of the STI layer204, side surfaces 211 of the channel structures 206, as well was topsurfaces 223 of the trench material 215 are exposed. The top surfaces223 of the trench material 215 and the side surfaces 211 of the channelstructures 206 may provide nucleation surfaces for subsequent epitaxialgrowth of semiconductor material. In some embodiments, the top surface223 of the trench material 215 is substantially coplanar with the topsurface 205 of the STI layer 204 as depicted in FIG. 2H. In alternativeembodiments, however, the top surface 223 of the trench material 215 isnot substantially coplanar with the top surface 205 of the STI layer204. As such, the top surface 223 of the trench material 215 may belower or higher than the top surface 205 of the STI layer 204. In anembodiment where the top surface 223 of the trench material 215 ishigher than the top surface 205 of the STI layer 204, the trenchmaterial 215 is directly adjacent to the channel structure 206. In analternative embodiment where the top surface 223 of the trench material215 is lower than the top surface 205 of the STI layer 204, the trench207 contains a semiconductor material different from the trench material215. For example, the trench material 215 composed of GaN may be etchedout of the trench 207 and subsequently filled with InGaN. In any case,the semiconductor material in the trench 207 provides a nucleationsurface for epitaxial growth of semiconductor material. Any suitablemasking and etching process may be used to remove defective portions ofthe blanket layer 216. In one embodiment, a dry etch process that usesCl₂ plasma is used to remove defective portions of the blanket layer 216and 206.

Thereafter, as shown in FIG. 2I, a semiconductor material for a source208 and a drain 210 is epitaxially grown within the openings formed inthe blanket layer 216 and 206. As such, the source 208 and drain 210 maybe self-aligned to the channel structure 206. The exposed top surface223 of the trench material 215 and side surfaces 211 of the channelstructure 206 serve as nucleating surfaces from which the source 208 anddrain 210 can epitaxially grow. Furthermore, the insulating layer 228prevents epitaxial growth of semiconductor material from the top of thechannel structure 206. As such, the source 208 and drain 210 form onopposite sides of the channel structure 206. The drain 210 may bedisposed directly on the top surface 205 of the STI layer 204 and thesource 208 may be formed directly on the top surface 223 of the trenchmaterial 215 to form a source-in-trench structure. Nucleation of thesource 208 on the trench material 215 may thermally couple the source208 with the substrate 202 via the trench material 215. Direct thermalcommunication between the source 208 and the substrate 202 may providedirect heat sinking to the substrate 202, advantageously enhancingthermal dissipation during transistor operation. Although the source 208forms on the trench material 215 in some embodiments, alternativeembodiments may invert the locations of the source 208 and drain 210.

The source 208 and drain 210 may be composed of a differentsemiconductor material than the channel structure 206. Although theirsemiconductor materials may be different, the lattice structure of thesource 208 and drain 210 may be similar to the channel structure 206such that epitaxial growth is possible. In an embodiment, the source 208and drain 210 may be composed of a semiconductor material whose overalleffective band gap is narrower than the channel structure 206 in orderto minimize contact resistance with a first contact (not shown). In anembodiment of the invention, the channel structure 206 is composed ofGaN and the source 208 and drain 210 are composed of a GaN alloy, suchas InGaN. The source 208 and drain 210 may be formed by any well-knownepitaxial growth technique, such as VPE, MBE, and CVD.

According to an embodiment, the narrower band gap of the semiconductormaterial used to form the source 208 and drain 210 is obtained byincreasing the amount of the more conductive element. For instance, inan embodiment, InN is alloyed with GaN to form an InGaN alloy for thesource 208 and drain 210. Because InN has a band gap of 0.9 eV, as theamount of In in the InGaN alloy increases, the overall effective bandgap of the InGaN decreases. In a particular embodiment, theconcentration of In is such that the overall effective band gap of InGaNranges from 1.5 eV to 3.2 eV. Furthermore, the crystalline structure ofInGaN is similar to GaN. As such, the InGaN alloy can be epitaxiallygrown from the side surfaces 211 of the channel structure 206 comprisingGaN while maintaining a narrow band gap for minimizing contactresistance with a first contact (not shown). The source 208 and drain210 may also be composed of a highly doped N⁺ semiconductor material inorder to further reduce their overall effective band gap. Any suitableN⁺ dopant, such as silicon, may be introduced during the epitaxialgrowth of the source 208 and drain 210 to form the highly doped N⁺semiconductor material. It is to be appreciated that as silicon dopantsincrease to concentrations well over 5E19 cm⁻³, the process of formingthe source 208 and drain 210 become more akin to blanket deposition thanepitaxial growth. As such, one skilled in the art may adjust the dopantconcentration to maximize N⁺ dopant concentration while maintainingepitaxial growth of the source 208 and drain 210. In one embodiment, thedoping concentration of the highly doped N⁺ semiconductor material is ashigh as 5E19 cm⁻³. In an alternative embodiment, the dopingconcentration of the highly doped N⁺ semiconductor material results in asemiconductor material with a sheet resistance of 40 to 50 ohm/sq.

Unlike the channel structure 206, the source 208 and drain 210 merelyact as nodes for current flow. As such, the source 208 and drain 210 maynot require such low defect densities as the channel structure 206. Inembodiments, the source 208 and drain 210 have a defect density greaterthan 1E9 cm⁻².

Next, as shown in FIG. 2J, a gate stack including a gate dielectric 213and a gate electrode 212 is formed on top of the channel structure 206.Any suitable material, such as silicon dioxide, may be used to form thegate dielectric 213. In an embodiment, the insulating layer 228 is usedas the gate dielectric 213. In alternative embodiments, the dielectriclayer 228 is removed and a gate dielectric 213 is formed. The gateelectrode 212 can be composed of any suitable material, such aspolysilicon. The gate electrode 212 and the gate dielectric 213 can beformed by any deposition and etch technique well-known in the art.

In FIG. 2K, gate spacers 214 may subsequently be formed on oppositesides of the gate stack 212 and 213. The gate spacers 214 may becomposed of any suitable spacer material, such as silicon dioxide,silicon nitride, or silicon carbide. Furthermore, any suitable spaceretching process may be used to form the gate spacer 214.

FIG. 3 illustrates a computing system 300 implemented with oneimplementation of the invention. The computing device 300 houses a board302. The board 302 may include a number of components, including but notlimited to a processor 304 and at least one communication chip 306. Theprocessor 304 is physically and electrically coupled to the board 302.In some implementations the at least one communication chip 306 is alsophysically and electrically coupled to the board 302. In furtherimplementations, the communication chip 306 is part of the processor304.

Depending on its applications, computing device 300 may include othercomponents that may or may not be physically and electrically coupled tothe board 302. These other components include, but are not limited to,volatile memory (e.g., DRAM), non-volatile memory (e.g., ROM), flashmemory, a graphics processor, a digital signal processor, a cryptoprocessor, a chipset, an antenna, a display, a touchscreen display, atouchscreen controller, a battery, an audio codec, a video codec, apower amplifier, a global positioning system (GPS) device, a compass, anaccelerometer, a gyroscope, a speaker, a camera, and a mass storagedevice (such as hard disk drive, compact disk (CD), digital versatiledisk (DVD), and so forth).

The communication chip 306 enables wireless communications for thetransfer of data to and from the computing device 300. The term“wireless” and its derivatives may be used to describe circuits,devices, systems, methods, techniques, communications channels, etc.,that may communicate data through the use of modulated electromagneticradiation through a non-solid medium. The term does not imply that theassociated devices do not contain any wires, although in someembodiments they might not. The communication chip 306 may implement anyof a number of wireless standards or protocols, including but notlimited to Wi-Fi (IEEE 802.11 family), WiMAX (IEEE 802.16 family), IEEE802.20, long term evolution (LTE), Ev-DO, HSPA+, HSDPA+, HSUPA+, EDGE,GSM, GPRS, CDMA, TDMA, DECT, Bluetooth, derivatives thereof, as well asany other wireless protocols that are designated as 3G, 4G, 5G, andbeyond. The computing device 300 may include a plurality ofcommunication chips 306. For instance, a first communication chip 306may be dedicated to shorter range wireless communications such as Wi-Fiand Bluetooth and a second communication chip 306 may be dedicated tolonger range wireless communications such as GPS, EDGE, GPRS, CDMA,WiMAX, LTE, Ev-DO, and others.

The processor 304 of the computing device 300 includes an integratedcircuit die packaged within the processor 304. In some implementationsof the invention, the integrated circuit die of the processor includesone or more devices, such as wide band gap transistors formed onnon-native semiconductor substrates, that are formed in accordance withimplementations of the invention. The term “processor” may refer to anydevice or portion of a device that processes electronic data fromregisters and/or memory to transform that electronic data into otherelectronic data that may be stored in registers and/or memory.

The communication chip 306 also includes an integrated circuit diepackaged within the communication chip 306. In accordance with anotherimplementation of the invention, the integrated circuit die of thecommunication chip includes one or more devices, such as airgapinterconnects with hood layers, that are formed in accordance withimplementations of the invention.

In further implementations, another component housed within thecomputing device 300 may contain an integrated circuit die that includesone or more devices, such as airgap interconnects with hood layers, thatare formed in accordance with implementations of the invention.

In various implementations, the computing device 300 may be a laptop, anetbook, a notebook, an ultrabook, a smartphone, a tablet, a personaldigital assistant (PDA), an ultra mobile PC, a mobile phone, a desktopcomputer, a server, a printer, a scanner, a monitor, a set-top box, anentertainment control unit, a digital camera, a portable music player,or a digital video recorder. In further implementations, the computingdevice 300 may be any other electronic device that processes data.

In an embodiment, a semiconductor transistor structure comprises asemiconductor substrate; an insulating layer formed on top of thesilicon substrate; a trench extending through the insulating layer andinto the silicon substrate, the trench containing a trench materialcomprising a first III-V semiconductor material; a channel structureformed directly on top of the insulating layer and adjacent to thetrench, the channel structure formed with a channel material comprisinga second III-V semiconductor material having a defect density lower thana defect density of the trench material; a source and drain formed onopposite sides of the channel structure, the source formed on top of thetrench material; and a gate electrode formed above the channelstructure. In another embodiment, the trench material and the channelmaterial comprise gallium nitride. In yet another embodiment, thechannel material has a defect density less than 1E9 cm⁻². In analternative embodiment, the trench material has a defect density greaterthan 1E9 cm⁻². In another embodiment, the source is formed on top of thetrench material and the drain is formed on top of the insulating layer.In yet another embodiment, the source is thermally coupled with thetrench material. In an alternative embodiment, the source is thermallycoupled with the silicon substrate. In one embodiment, the source anddrain comprise indium gallium nitride. In one other embodiment, theindium gallium nitride has an N⁺ doping concentration higher than 5E19cm⁻³. In yet another embodiment, the semiconductor substrate comprisessilicon, the trench material and channel structure comprise GaN, and thesource and drain comprise InGaN.

In one embodiment, a method of forming a semiconductor transistorstructure comprises forming a patterned insulating layer on asemiconductor substrate, the patterned insulating layer exposing anuncovered portion of the semiconductor substrate; forming a trench inthe semiconductor substrate at the uncovered portion of thesemiconductor substrate; growing a semiconductor material within thetrench such that the material laterally overflows onto the patternedinsulating layer and forms a blanket layer, the material comprising aIII-V semiconductor material; etching away a portion of the blanketlayer such that a channel structure and a trench material remain, thechannel structure having a lower defect density than the trenchmaterial; forming a source and drain on opposite sides of the channelstructure; and forming a gate electrode on top of the channel structure.In an alternative embodiment, the blanket layer is formed by thematerial coalescing with an adjacent laterally overflowing material. Inanother embodiment, the forming a trench in the semiconductor substrateis a selective etching of the uncovered portion of the semiconductorsubstrate. In yet another embodiment, the material comprising a III-Vsemiconductor material is gallium nitride. In one embodiment, theportion of the blanket layer is a defective portion having a defectdensity greater than a defect density of the channel structure. In oneother embodiment, the defective portion contains a defect densitygreater than 1E9 cm⁻². In an alternative embodiment, the defect densityof the channel structure is less than 1E9 cm⁻². In another embodiment,the trench material has a defect density greater than 1E9 cm⁻². In yetanother embodiment, the source and drain are formed by epitaxial growth.In one embodiment, the epitaxial growth forms an indium gallium nitridestructure. In one other embodiment, the indium gallium nitride structureis formed with an N⁺ doping concentration higher than 5E19 cm⁻³.

In an embodiment, a system-on-chip comprises a semiconductor substrate;a metal oxide semiconductor transistor formed on the semiconductorsubstrate; and a wide band gap semiconductor transistor formed on thesemiconductor substrate and adjacent to the metal oxide semiconductortransistor, comprising an insulating layer formed on top of thesemiconductor substrate; a trench formed through the insulating layerand extending into the semiconductor substrate, the trench filled with atrench material comprising a III-V semiconductor material; a channelstructure formed directly on top of the insulating layer and adjacent tothe trench, the channel structure formed with a channel materialcomprising a III-V semiconductor material having a defect density lowerthan a defect density of the trench material; a source and drain formedon opposite sides of the channel structure; and a gate electrode formedon top of the channel structure. In an alternative embodiment, thetrench material and the channel material comprise gallium nitride. Inanother embodiment, the channel material has a defect density of lessthan 1E9 cm⁻². In yet another embodiment, the trench material has adefect density greater than 1E9 cm⁻². In one embodiment, the source isformed on top of the trench material and the drain is formed on top ofthe insulating layer. In one other embodiment, the source is thermallycoupled with the trench material. In a different embodiment, the sourceis thermally coupled with the semiconductor substrate. In anotherembodiment, the source and drain comprise indium gallium nitride. In yetanother embodiment, the indium gallium nitride has an N⁺ dopingconcentration higher than 5E19 cm⁻³.

In utilizing the various aspects of this invention, it would becomeapparent to one skilled in the art that combinations or variations ofthe above embodiments are possible for forming a wide band gaptransistor on a non-native semiconductor substrate. Although embodimentsof the present invention have been described in language specific tostructural features and/or methodological acts, it is to be understoodthat the invention defined in the appended claims is not necessarilylimited to the specific features or acts described. The specificfeatures and acts disclosed are instead to be understood as particularlygraceful implementations of the claimed invention useful forillustrating embodiments of the present invention.

What is claimed is:
 1. A semiconductor transistor structure, comprising:a silicon substrate; an insulating layer formed on top of the siliconsubstrate; a trench extending through the insulating layer and into thesilicon substrate, the trench containing a trench material comprising afirst III-V semiconductor material; a channel structure formed directlyon top of the insulating layer and adjacent to the trench, the channelstructure formed with a channel material comprising a second III-Vsemiconductor material having a defect density lower than a defectdensity of the trench material; a source and drain formed on oppositesides of the channel structure, the source formed on top of the trenchmaterial; and a gate electrode formed above the channel structure. 2.The semiconductor transistor structure of claim 1, wherein the trenchmaterial and the channel material comprise gallium nitride.
 3. Thesemiconductor transistor structure of claim 1, wherein the channelmaterial has a defect density less than 1E9 cm-2.
 4. The semiconductortransistor structure of claim 1, wherein the trench material has adefect density greater than 1E9 cm-2.
 5. The semiconductor transistorstructure of claim 1, wherein the source is formed on top of the trenchmaterial and the drain is formed on top of the insulating layer.
 6. Thesemiconductor transistor structure of claim 5, wherein the source isthermally coupled with the silicon substrate.
 7. The semiconductortransistor structure of claim 1, wherein the source and drain compriseindium gallium nitride.
 8. The semiconductor transistor structure ofclaim 7, wherein the indium gallium nitride has an N+ dopingconcentration higher than 5E19 cm-3.
 9. The semiconductor transistorstructure of claim 1, wherein the trench material and channel structurecomprise GaN, and the source and drain comprise InGaN.
 10. Asystem-on-chip, comprising: a semiconductor substrate; a metal oxidesemiconductor transistor formed on the semiconductor substrate; and awide band gap semiconductor transistor formed on the semiconductorsubstrate and adjacent to the metal oxide semiconductor transistor,comprising: an insulating layer formed on top of the semiconductorsubstrate; a trench formed through the insulating layer and extendinginto the semiconductor substrate, the trench filled with a trenchmaterial comprising a III-V semiconductor material; a channel structureformed directly on top of the insulating layer and adjacent to thetrench, the channel structure formed with a channel material comprisinga III-V semiconductor material having a defect density lower than adefect density of the trench material; a source and drain formed onopposite sides of the channel structure; and a gate electrode formed ontop of the channel structure.
 11. The semiconductor transistor structureof claim 10, wherein the trench material and the channel materialcomprise gallium nitride.
 12. The semiconductor transistor structure ofclaim 10, wherein the channel material has a defect density of less than1E9 cm-2.
 13. The semiconductor transistor structure of claim 10,wherein the trench material has a defect density greater than 1E9 cm-2.14. The semiconductor transistor structure of claim 10, wherein thesource is formed on top of the trench material and the drain is formedon top of the insulating layer.
 15. The semiconductor transistorstructure of claim 10, wherein the source is thermally coupled with thesemiconductor substrate.
 16. The semiconductor transistor structure ofclaim 10, wherein the source and drain comprise indium gallium nitride.